Yolk sac
Encyclopedia
The yolk sac is a membranous sac attached to an embryo
, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals. It functions as the developmental circulatory system
of the human embryo, before internal circulation begins.
during pregnancy
, usually at 5 weeks gestation
.
It is a critical landmark, identifying a true gestation sac.
It is reliably seen early in human pregnancy using ultrasound
.
The yolk sac is situated on the ventral aspect of the embryo
; it is lined by extra-embryonic endoderm
, outside of which is a layer of extra-embryonic mesenchyme, derived from the mesoderm
.
Blood is conveyed to the wall of the sac by the primitive aorta
, and after circulating through a wide-meshed capillary plexus, is returned by the vitelline veins
to the tubular heart of the embryo. This constitutes the vitelline circulation
, and by means of it nutritive material is absorbed from the yolk sac and conveyed to the embryo.
At the end of the fourth week the yolk sac presents the appearance of a small pear-shaped vesicle (umbilical vesicle) opening into the digestive tube by a long narrow tube, the vitelline duct
.
The vesicle can be seen in the afterbirth as a small, somewhat oval-shaped body whose diameter varies from 1 mm. to 5 mm.; it is situated between the amnion
and the chorion
and may lie on or at a varying distance from the placenta
.
As a rule the duct undergoes complete obliteration during the seventh week, but in about two percent of cases its proximal part persists as a diverticulum from the small intestine, Meckel's diverticulum
, which is situated about 60 cm proximal to the ileocecal valve
, and may be attached by a fibrous cord to the abdominal wall at the umbilicus
.
Sometimes a narrowing of the lumen of the ileum
is seen opposite the site of attachment of the duct.
s and rays have yolk sacs at some stage of development, with all oviparous fished retaining the sac after hatching. Lamniform sharks
are ovoviviparous, in that their eggs hatch in utero, in addition to eating unfertilized eggs, unborn sharks participate in intrauterine-cannibalism: stronger pups consume their weaker womb-mates.
starts proliferating laterally and descending.
In the meantime the Heuser membrane, located on the opposite pole of the developing vesicle, starts its upward proliferation and meets the hypoblast.
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals. It functions as the developmental circulatory system
Erythropoiesis
Erythropoiesis is the process by which red blood cells are produced. It is stimulated by decreased O2 in circulation, which is detected by the kidneys, which then secrete the hormone erythropoietin...
of the human embryo, before internal circulation begins.
In humans
It is the first element seen in the gestational sacGestational sac
The gestational sac is the only available intrauterine structure that can be used to determine if an intrauterine pregnancy exists, until the embryo is identified....
during pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
, usually at 5 weeks gestation
Gestation
Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....
.
It is a critical landmark, identifying a true gestation sac.
It is reliably seen early in human pregnancy using ultrasound
Ultrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...
.
The yolk sac is situated on the ventral aspect of the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
; it is lined by extra-embryonic endoderm
Endoderm
Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and mesoderm , with the endoderm as the intermost layer...
, outside of which is a layer of extra-embryonic mesenchyme, derived from the mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
.
Blood is conveyed to the wall of the sac by the primitive aorta
Aorta
The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...
, and after circulating through a wide-meshed capillary plexus, is returned by the vitelline veins
Vitelline veins
-Path:They run upward at first in front, and subsequently on either side of the intestinal canal.They unite on the ventral aspect of the canal, and beyond this are connected to one another by two anastomotic branches, one on the dorsal, and the other on the ventral aspect of the duodenal portion of...
to the tubular heart of the embryo. This constitutes the vitelline circulation
Vitelline circulation
Vitelline circulation refers to the system of blood flowing from the embryo to the yolk sac and back again.The yolk-sac is situated on the ventral aspect of the embryo; it is lined by endoderm, outside of which is a layer of mesoderm...
, and by means of it nutritive material is absorbed from the yolk sac and conveyed to the embryo.
At the end of the fourth week the yolk sac presents the appearance of a small pear-shaped vesicle (umbilical vesicle) opening into the digestive tube by a long narrow tube, the vitelline duct
Vitelline duct
In the human embryo, the vitelline duct, also known as the omphalomesenteric duct, is a long narrow tube that joins the yolk sac to the midgut lumen of the developing fetus...
.
The vesicle can be seen in the afterbirth as a small, somewhat oval-shaped body whose diameter varies from 1 mm. to 5 mm.; it is situated between the amnion
Amnion
The amnion is a membrane building the amniotic sac that surrounds and protects an embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibians and fish , which are consequently termed “Anamniota”. The primary role of this is the protection of the...
and the chorion
Chorion
The chorion is one of the membranes that exist during pregnancy between the developing fetus and mother. It is formed by extraembryonic mesoderm and the two layers of trophoblast and surrounds the embryo and other membranes...
and may lie on or at a varying distance from the placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
.
As a rule the duct undergoes complete obliteration during the seventh week, but in about two percent of cases its proximal part persists as a diverticulum from the small intestine, Meckel's diverticulum
Meckel's diverticulum
A Meckel's diverticulum, a true congenital diverticulum, is a small bulge in the small intestine present at birth. It is a vestigial remnant of the omphalomesenteric duct , and is the most frequent malformation of the gastrointestinal tract...
, which is situated about 60 cm proximal to the ileocecal valve
Ileocecal valve
The ileocecal valve, or ileocaecal valve, is of a bilabial papilla structure with physiological sphincter muscle situated at the junction of the small intestine and the large intestine, with recent evidence indicating an anatomical sphincter may also be present in humans) Its critical function is...
, and may be attached by a fibrous cord to the abdominal wall at the umbilicus
Navel
The navel is a scar on the abdomen caused when the umbilical cord is removed from a newborn baby...
.
Sometimes a narrowing of the lumen of the ileum
Ileum
The ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum.The ileum follows the duodenum...
is seen opposite the site of attachment of the duct.
In fish
All bony fishes, some sharkShark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s and rays have yolk sacs at some stage of development, with all oviparous fished retaining the sac after hatching. Lamniform sharks
Lamniformes
Lamniformes is an order of sharks commonly known as mackerel sharks . It includes some of the most familiar species of sharks, such as the great white shark, as well as more unusual representatives, such as the goblin shark and the megamouth shark.Members of the order are distinguished by...
are ovoviviparous, in that their eggs hatch in utero, in addition to eating unfertilized eggs, unborn sharks participate in intrauterine-cannibalism: stronger pups consume their weaker womb-mates.
Histogenesis
The yolk sac starts forming itself during the second week of the embryonic development, at the same time of the shaping of the amniotic sac. The hypoblastHypoblast
The hypoblast is a tissue type that forms from the inner cell mass. It lies beneath the epiblast and consists of small cuboidal cells.Extraembryonic endoderm is derived from hypoblast.-External links:...
starts proliferating laterally and descending.
In the meantime the Heuser membrane, located on the opposite pole of the developing vesicle, starts its upward proliferation and meets the hypoblast.
Modifications
- Primary yolk sac/primitive yolk sac: it is the vesicle constituted in the second week, its floor is represented by the Heuser membrane and its ceiling by the hypoblastHypoblastThe hypoblast is a tissue type that forms from the inner cell mass. It lies beneath the epiblast and consists of small cuboidal cells.Extraembryonic endoderm is derived from hypoblast.-External links:...
. It is also known as the exocoelomic cavity.
- Secondary yolk sac: this first transformation is determined by the modification of its cover, in the connection zone between the epiblast and the Heuser membrane. We can observe a structure. The two parts detach and the inferior one, which is smaller, forms a cyst destined to be eliminated. The upper one is now covered only by the epiblast.
- The final yolk sac: during the fourth week of development, during which we can see the shaping of the embryonic areas. A little portion of the sac, in the upper part, constitutes the intestinal tube. On the other side, the distal part forms a little vesicle that is what remains of the yolk sac.